1. Field of the Invention
The present invention relates generally to cardiac pacemakers, and more particularly to an exercise-responsive implantable cardiac pacemaker in which the stimulation rate is adaptively regulated according to the blood temperature of the pacemaker patient.
2. Prior Art
In situations where the natural pacemaker or pacing system of a patient's heart is disturbed because of age, disease or injury, it is customary to employ artificial pacing of the heart by implanting a cardiac pacemaker. In an atrial-triggered pacemaker, the P-wave generated preceding atrial contraction is detected to initiate the delivery of a pacing stimulus to the ventricle. It has been found that an atrial-triggered pacemaker is, to an extent, responsive to physical exertion of the patient, unlike the conventional fixed-rate pacemaker. However, in many cardiac patients, such as those suffering from atrial flutter, fibrillation, or sick-sinus syndrome, P-wave generation is not responsive to physiological conditions. Hence, the exercise-responsive advantage of atrial triggered pacemakers is not available to such patients.
In the past, many proposals have been advanced for adapting the pacemaker stimulation rate to patient exercise using a detected biological signal. Biological parameters proposed as suited for exercise-responsive adjustment of pacing rate include, for example, the pH value of the venous blood, the central venous oxygen saturation, the respiration rate, the Q-T interval (i.e., the interval from ventricular depolarization to repolarization), and the central venous blood temperature.
Cardiac pacemakers using the respiration rate or the Q-T interval for pacing rate control are currently in development and/or undergoing clinical testing. However, the use of the Q-T interval can easily cause oscillations, and thus, pacemaker-triggered tachycardia. Furthermore, the parameter these devices employ for rate control makes them particularly subject to disruption by medications currently in use to act on the electrolyte or membrane metabolism, such as beta-blocking agents, diuretics, antiarrhythmics, and digitalis.
As pointed out in publication OS 26 09 365 of the Federal Republic of Germany, dated Sept. 8, 1977, the central venous blood temperature may serve as a biological parameter for controlling or regulating the stimulation rate of a rate-adjustable cardiac pacemaker. A temperature-controlled pacemaker as described in that publication has not, to my knowledge, yet been used in actual practice. It does offer the advantage of employing a relatively Csmall and simple temperature sensor that may be incorporated in the catheter lead in proximity to the pacing electrode. The aforementioned publication proposes that the stimulation rate be adjusted in parallel with the blood temperature; that is to say, a rise in blood temperature would provide a correspondingly higher stimulation rate, not excluding a linear dependency between the two in an exemplary partial range of from 37.degree. to 39.degree. C.
Another rate-adaptive cardiac pacemaker similarly depending on central venous blood temperature has been proposed in U.S. Pat. No. 4,436,092, issued Mar. 13, 1984. According to that disclosure, the blood temperature is detected by a thermistor mounted on the same lead as the stimulating electrode, with the lead introduced intravenously such that the thermistor is positioned within the right ventricle of the heart. An exercise algorithm based on an observed mathematical relationship between blood temperature and heart rate in a normally functioning heart under stress, and in which constants are derived from experimental data pertaining to the particular patient who will be using the pacemaker, is utilized in conjunction with the the thermistor signal to control the pulse frequency of the pacemaker's pulse generator. This type of control appears to be similar to the general control principle proposed by Csapo et al. in the aforementioned German publication OS 26 09 365, and to the presentation by the latter at the VIII World Congress of Cardiology in Tokyo, Sept. 17-23, 1978, as detailed in the presentation manuscript. However, neither of these previously proposed approaches provides optimal adaptation of stimulation rate to the physiological condition of the cardiac pacemaker patient. In particular, the system described in the aforementioned U.S. patent employs a given relationship of blood temperature to heart rate in which heart rate is based on the summation of a base rate with a temperature dependent higher rate. Accordingly, the rate-adaptive pacemaker proposed therein provides only one single relationship between the stimulation rate and the instantaneous blood temperature.